Multi-carrier data communication systems transmit data by splitting it into several components and sending each of these components over separate carrier signals. The communication channel, which generally has broad bandwidth, is divided into sub-channels that generally have narrow bandwidth. The individual carrier is generally at the center of the sub-channel. In Asymmetric Digital Subscriber Line (ADSL) systems, for example, the number of sub-channels is typically 128 or 256.
When data is to be sent over the communication channel, it is split into several groups of small numbers of bits, with each of the groups of bits being modulated onto the carrier of a sub-channel and transmitted in parallel. The allocation of the data bits among the sub-channels is known collectively as the “bit loading” of the channel, while the assignment of the relative gains applied to the transmitted symbols of the sub-channels is known as the “energy loading”. The bit loading and energy loading are calculated according to a bit-loading algorithm, several of which are known in the art. The bits are generally not allocated evenly among all the sub-channels. Those sub-channels having one or more bits allocated to them are called “active” sub-channels.
Given a particular bit loading, the total bit rate for the channel is the product of the baud rate (composite symbols per second) and the total number of allocated bits. The maximum number of bits that any particular sub-channel can have allocated thereto may depend on a number of factors, such as sub-channel conditions (for example, the signal-to-noise ratio of the sub-channel), system configuration options and specified system performance goals. The maximum achievable total bit rate is then the product of the baud rate and the sum of the maximum number of bits that each sub-channel can carry.
There are several reasons that the target bit rate may be lower than the maximum achievable total bit rate. In a non-limiting example, in ADSL, the central office may limit the total bit rate to a much lower rate than could be achieved. In another non-limiting example, the total bit rate may be reduced in order to maintain a total power constraint. In a further non-limiting example, the total bit rate may be temporarily reduced during periods of little or no activity at the communication layers or the application layer above the physical layer.
In some multi-carrier data communication systems, the processing power available to modulate and/or demodulate data may be limited.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.